1. Field of the Invention
The present invention relates generally to a channel communication apparatus and method in a CDMA communication system, and in particular, to an apparatus and method for assigning codes necessary for synchronization and channel separation using an uplink synchronous transmission scheme in a CDMA communication system.
2. Description of the Related Art
A CDMA (Code Division Multiple Access) system is divided into a synchronous system and an asynchronous system. Such a CDMA communication system uses orthogonal codes to separate channels. Herein, a description of the invention will be made with reference to an asynchronous W-CDMA (Wideband-CDMA) communication system, also known as a UMTS (Universal Mobile Telecommunications System) communication system. However, the invention can also be applied to a different CDMA system such as the CDMA-2000 system, as well as the W-CDMA system.
FIG. 1 illustrates architecture of a W-CDMA communication system. As illustrated, the W-CDMA communication system includes an RNC (Radio Network Controller) 101, a plurality of Node B's 103-1 to 103-3 (hereinafter, referred to as a “UTRAN” (UMTS Terrestrial Radio Access Network); a base station transceiver subsystem (BTS) in the CDMA-2000 system) connected to the RNC 101, and a user equipment (UE; a mobile station in the CDMA-2000 system) 105 connected to one of the UTRANs 103-1 to 103-3 through a radio interface. Every process for connection of the UE 105 is controlled by the RNC 101. Further, the RNC 101 manages assignment of channel resources to the UEs connected to the UTRANs.
When successfully connected to the UTRAN through the channel assigned by the RNC 101, the UE 105 maintains communication using the assigned downlink or uplink dedicated physical channel (DPCH). The W-CDMA communication system uses the asynchronous channels, which are not synchronized with one another. In this case, the UTRAN 103 assigns a unique scrambling code to the UE 105 in order to distinguish one UE 105 from another UE.
The scrambling code is classified into a long scrambling code and a short scrambling code. In the following description, the “scrambling code” will refer to the long scrambling code.
The scrambling code is created in the following process of:
(Step 1) receiving 24 initial values n0,n1, . . . ,n23,
(Step 2) creating sequences x(i) and y(i), where i=0, . . . , 225−27,                x(0)=n0, x(1)=n1, x(2)=n2, . . . , x(23)=n23, x(24)=1        x(i+25)=x(i+3)+x(i) modulo 2, i=0, . . . , 225−27        y(0)=y(1)=y(2)= . . . =y(23)=y(24)=1        y(i+25)=y(i+3)+y(i+2)+y(i+2)+y(i) modulo 2, i=0, . . . , 225−27        
(Step 3) creating a sequence z(i), where i=0, . . . , 225−2,                z(i)=x(i)+y(i) modulo 2, i=0, . . . , 225−2,        
(Step 4) creating a Gold sequence Z(i), where i=0, . . . , 225−2,                Z(i)=1–2*z(i)        
(Step 5) creating two real scrambling codes c1(i) and c2(i), where i=0, . . . , 225−2,                c1(i)=Z(i)        c2(i)=Z((i+16777232) modulo (225−1)),        
(Step 6) creating a scrambling code C(i), where i=0, . . . , 225−2,                C(i)=c1(i)*(1+j(−1)i*c2(2*┌i/2┐)).        
In the above formula, ┌x┐ indicate the largest one of integers smaller than or equal to a value x.
The scrambling codes created in this method are assigned to the UEs by the UTRAN during assignment of the DPCHs, in order to distinguish the UEs from one another.
In the W-CDMA communication system, one frame is comprised of 38400 chips. Therefore, the scrambling code is used in a unit of 38400 chips, and this can be achieved by using part of the scrambling code. That is, a scrambling code for one DPCH is C(i), where i=0,1, . . . ,38399.
The DPCHs use scrambling codes starting from C(0) at the start point of the frame. The respective DPCHs have different initial values n0,n1, . . . ,n23, so that different scrambling codes are assigned to the DPCHs.
The latest W-CDMA communication system uses OVSF (Orthogonal Variable Spreading Factor) codes for channel separation. The OVSF codes maintain an orthogonal property among different physical channels of the UEs, thereby to identify the channels assigned to the UEs. Further, in the downlink, the OVSF codes can have different rates by varying the length of the OVSF code, i.e., spreading factor (SF). In the uplink, the OVSF codes are used to distinguish the channels used by one UE. In an uplink synchronous transmission scheme (USTS) where the UEs use the same scrambling code, the OVSF codes are used to distinguish the channels of the UEs.
The latest W-CDMA communication system maintains the time-asynchronous property by giving different time offsets to the DPCHs. This is because the downlink DPCH (DL DPCH) has a control part at its head. That is, the control parts of the downlink DPCHs are transmitted at a different time so as to resolve a power problem that may occur when the control parts are simultaneously transmitted. In addition, the uplink DPCH (UL DPCH) also has frame ends, that arrive at the Node B at different times so as to minimize any influence on the processing speed of the UTRAN.
FIG. 2 illustrates the timing relationship between the downlink DPCH and the uplink DPCH in the W-CDMA communication system. An asynchronous transmission scheme for the downlink DPCH and the uplink DPCH will be described below with reference to FIG. 2.
Referring to FIG. 2, one 10-ms frame is comprised of 15 slots, and each slot is comprised of 2560 chips. In FIG. 2, reference numeral 201 indicates a common pilot channel (CPICH), reference numeral 203 indicates a primary common control physical channel (P-CCPCH), reference numeral 205 indicates a frame in a slot unit, reference numerals 207 and 209 indicate downlink DPCHs, and reference numerals 208 and 210 indicate uplink DPCHs associated respectively with the downlink DPCHs 207 and 209. The CPICH 201 and the P-CCPCH 203 undergo frame synchronization, and are used as reference channels for other channels.
As illustrated in FIG. 2, the downlink DPCHs 207 and 209 are transmitted with time offsets τDPCH,n and τDPCH,n+1 from the P-CCPCH 203, respectively. As stated above, the DPCHs are given the different time offsets τDPCH. For example, each DPCH is given one of 0, 256, 2*256, . . . , 148*256 and 149*256-chip offsets.
After a lapse of To time upon receiving the downlink DPCHs 207 and 209 transmitted with the time offsets τDPCH,n and τDPCH,n+1 to the P-CCPCH 203, the UE transmits the associated uplink DPCHs 208 and 210. Therefore, the uplink DPCHs 208 and 210 are also synchronized with each other. Due to a distance difference between the UTRAN and the respective UEs, the UTRAN may not receive the uplink DPCH exactly the To time after transmitting the downlink DPCH associated with the uplink DPCH. Therefore, the UTRAN measures a propagation delay time to the UE in the process of transmitting a random access channel (RACH) in order to measure a distance from the UE, and uses this value for initial synchronization. That is, the UTRAN uses the propagation delay time in predicting an expected uplink DPCH reception time after transmission of the downlink DPCH.
An uplink synchronous transmission scheme (USTS) assigns a single scrambling code to several UEs. The USTS is designed to synchronize the uplink DPCHs when the UTRAN receives the uplink DPCHs transmitted from the UEs. By using the USTS, the UTRAN can assign the same single scrambling code to the synchronized UEs. Therefore, the W-CDMA communication system employing the USTS can reduce the number of scrambling codes used in the cell, contributing to a reduction in interference between UE signals. When the UEs employing the USTS use the same scrambling code, the UTRAN can identify the UEs using channelization codes, i.e., the OVSF orthogonal codes provided from the RNC. In the USTS, the UTRAN synchronizes the uplink DPCHs of at least 2 UEs with each other, and then assigns the same scrambling code to the synchronized UEs. Further, the UTRAN assigns the different channelization codes (or OVSF codes) to the uplink DPCHs of the UEs assigned the same scrambling code, thereby to identify the received synchronized uplink DPCHs.
The USTS controls a sync time of the signal through the following two processes.
(1) Initial Synchronization Process
Upon receipt of a signal from the UE over the RACH, the UTRAN measures a difference between a predetermined reference time and a reception time of the signal received over the RACH, i.e., measures a delay time of the RACH signal. The UTRAN transmits the time difference to the UE over a forward access channel (FACH). Upon receipt of the time difference over the FACH, the UE aligns (or adjusts) a transmission time using the received time difference.
(2) Tracking Process
The UTRAN periodically transmits a time alignment bit to the UE through a comparison of the reception time of the UE signal and the reference time. If the time alignment bit is ‘1’, the UE shifts the transmission time by ⅛ chip ahead. However, if the time alignment bit is ‘0’, the UE shifts the transmission time by ⅛ chip behind. The time alignment bit is transmitted once every two frames using a transmit power control (TPC) bit in the control channel.
In the USTS where several UEs use the same single scrambling code, the UEs using the same scrambling code must be inevitably synchronized with one another. That is, when the UTRAN receives the DPCHs transmitted from several UEs, the received DPCHs must be subjected to both slot synchronization and frame synchronization. The frame synchronization is to minimize interference among the UEs using the same scrambling code, while slot synchronization is to distinguish the UEs using the same scrambling code by using the OVSF codes. The Initial Synchronization Process is a process for acquiring the frame synchronization and the slot synchronization.
As described above, the respective downlink DPCHs have a unique time offset τDPCH,n. Therefore, the uplink DPCHs are not synchronized with one another. In the Initial Synchronization Process, the mis-synchronization among the uplink DPCHs must be aligned to acquire synchronization. Therefore, there is a demand for a concrete method for resolving the channel mis-synchronization problem in the Initial Synchronization Process.